Domain Design: Multi-Forest
This design, while administratively more complex, provides the best security. It also raises support costs and makes collaboration a little more difficult, but it definitely has its benefits. This design will have standalone forests for all of the business units or departments. This also means that by default they cannot see or access each other. Administrators then create trust relationships between the different domains that are within the forests. This will give the granularity needed. To visually understand this, please see the following image:
